1. Modular approaches In file system development
The problem of creeping featurism and
upgrading the old abstraction basis

2. Edward Shishkin edward.shishkin@gmail.com edward@redhat.com

3. VFSw (Sun Microsystems, 1985)
Logical FS
sys_mount()
sys_open()
sys_read()
sys_write()
....
VFS
file_operations
inode_operations
address_space_operations FS
VFSw: Virtual File System switches

4. Switches (example) Name resolution root while() { ...
inode_ops = get_inode_ops();
inode_ops->lookup(); }
root
ext4
xfs

5. Extended Attributes (Xattrs)
VFS FS
xattrs

6. Journalling
VFS FS
xattrs
journalling

7. Extents and delayed allocation
VFS FS
journalling
xattrs
extents
delayed
allocation

8. Tail packing VFS xattrs journalling FS extents delayed allocation

9. Tail<->extent conversion
VFS FS
journalling
xattrs
extents
delayed
allocation
t-e conversion
tail packing

10. Delayed balancing VFS xattrs journalling FS extents delayed allocation
t-e conversion
tail packing
delayed balancing

11. Transparent compression
VFS FS
journalling
xattrs
extents
delayed
allocation
t-e conversion
tail packing
compression
delayed balancing

12. Transparent encryption
VFS FS
journalling
xattrs
extents
delayed
allocation
t-e conversion
tail packing
compression
encryption
delayed balancing

13. Delay of actions Action What is affected external fragmentation,
performance
1. Allocation
internal fragmentation,
performance
2. Balancing
3. Compression
performance
4. Encryption
performance

14. Sub-volumes VFS xattrs journalling FS extents delayed allocation
t-e conversion
tail packing
compression
delayed balancing
encryption
sub-volumes

15. Sub-volumes A B C D A' B' C' D'

16. Snapshots and WA-logging
VFS FS
xattrs
journalling
extents
delayed
allocation
t-e conversion
tail packing
compression
delayed balancing
WA-logs
encryption
sub-volumes
snapshots

17. Logging techniques Journal I. Journalling (overwrite) Permanent
location
New
permanent
location
II. WA-logging
(relocate)

18. Checksums VFS xattrs journalling FS checksums extents delayed
allocation
t-e conversion
tail packing
compression
delayed balancing
WA logs
encryption
sub-volumes
snapshots

19. Data de-duplication VFS xattrs journalling FS checksums extents
delayed
allocation
t-e conversion
tail packing
compression
delayed balancing
WA logs
encryption
sub-volumes
snapshots
data de-dup

20. Raid and LVM in FS layer FS VFS xattrs journalling checksums delayed
allocation
extents
t-e conversion
tail packing
compression
delayed balancing
encryption
WA logs
sub-volumes
snapshots
data de-dup
raid
LVM

21. 1) How to maintain? 2) How to change?
VFS FS
xattrs
journalling
checksums
delayed
allocation
extents
t-e conversion
tail packing
compression
delayed balancing
encryption
COW
sub-volumes
snapshots
data de-dup
raid
LVM

22. Possible ways to implement a new feature in the FS layer:
Write a new FS from scratch
Add this feature to an existing FS
Possible ways to implement a new feature in the FS layer:

23. Cost of a modern local FS
Kernel module: 25 human years
Fsck and user-space utilities: 8 human years
Conclusion: We should refuse the first way:
writing FS from scratch is too expensive
(and the cost tends to increase with years)!

24. How FS should be designed to minimize overheads when adding a new feature? To answer on this question we need to possess a deep understanding of what a file system feature is. And some progress in this direction is possible with 2 different approaches: Approach #1: DAGI, plugins (Reiser4) Approach #2: Translators (GlusterFS)

25. Introducing a new interface is not overdesign if at least one of the following takes place:
encapsulating disk formats
switches
transitions
chances that it will be used by other subsystems

26. VFS
TREE
->search_by_key();
->insert_object(balance_flags);
->cut_object(balance_flags);
->balance(); FS
TREE

27. VFS
NODE
->lookup();
->create_item();
->delete_item();
->shift();
... FS
TREE
ITEM
->lookup();
->number_of_units();
->mergeable(this, neighbor);
->read();
->write();
->read_pages();
...
ITEM
NODE

28. Managing transactions
VFS
TXM
->init_context();
->exit_context();
->capture_block();
... FS
TXM
TREE
ATOM
ATOM
->mark_block_dirty();
->flush();
->commit();
...
ITEM
NODE
Managing transactions

29. Delayed actions VFS SPACE ->alloc_extent(); ->free_extent(); ...
TXM
TREE
ATOM
COMPRESS
->compress();
->decompress();
...
ITEM
NODE
COMPRESS
SPACE
Delayed actions

30. Snapshots SNPM ->create_subvolume(); ... VFS FS TXM SNPM TREE ATOM
ITEM
NODE
NODE
COMPRESS
SPACE
Snapshots

31. Trivially graded DAGI-Graded (old abstraction, Sun 1985)
(Namesys, 2004)
VFS
VFS FS FS
TXM
SNPM
TREE
ATOM
ITEM
NODE
COMPRESS
SPACE
DAGI: Direct Acyclic Graph of Interfaces with weak CS relationships

32. Polymorphism of zero orderX
Polymorphism of zero order C Z
x->y Y A C1 X C2
Polymorphism of first order Z C3
x->x
z->x
z->y Y
B->A

33. GlusterFS: Client Server chains of interfaces withX1
VFS X2
Client FS
... Xn
Xn+1
Server
GlusterFS:
chains of interfaces with
strong CS-relationships
...